A number of network technologies have been developed for connecting the so-called “last mile” between a central office and subscriber. One such development is the passive optical network (PON). PONs typically include a fiber optic network between a central office (CO) and a subscriber comprising active network devices only at the CO and at the subscriber premises. As such, PONs generally require less power to operate, are more reliant, and can be upgraded without having to upgrade the plant between the CO and the subscriber.
PONs often are used to provide multiple types of data content, such as voice, data, and video, over the same network. To properly distribute this content, a number of common network protocols, such as Ethernet and Asynchronous Transfer Mode (ATM), are used to deliver the content over the PON. ATM PONs, or APONs, are particularly well suited for delivering real-time content, such as voice or videoconferencing, due to Quality of Service (QoS), small cell size, and other features incorporated by the ATM protocol. A specification for APONs has been adopted by the International Telecommunication Union (ITU) as Recommendations G.983.1, G.983.2, G.983.3, G.983.4, and G.983.5 (collectively known herein as the ITU G.983.X Recommendation). These recommendations address APON systems with symmetrical line rates of 155.520 Mbps and asymmetrical line rates of 155.520 Mbps upstream and 622.080 Mbps downstream. The recommendations also cover the physical layer requirements and specifications for the physical media dependent layer for an APON range up to 20 km (12.4 miles), the trans-convergence (TC) layer, security, and a ranging protocol. Additionally, dynamic bandwidth allocation (DBA) and data protection mechanisms are outlined.
Referring now to FIG. 1, an exemplary implementation of a known PON is illustrated. The known system 100 includes a central office 104 having an optical line termination (OLT) 110 connected to a number of optical network terminations (ONTs) 130-134 via a PON 120. Data, video, and/or voice content from various content providers is delivered to the OLT 110 of the CO 104. The OLT 110 typically is a component of an access multiplexer shelf that terminates the optical network in the CO 104. It receives and transmits APON optical signals via a fiber management shelf utilized to route between access multiplexer shelves and the outside fiber plant (PON 120). An optical module of the OLT 110 performs optical filtering, electronic-to-optical (E/O) conversion, and optical-to-electrical (O/E) conversion. The upstream data (i.e., from the subscriber devices to a content provider via the OLT 110) is de-framed, OAM extracted, and upstream data multiplexed with other upstream data before being sent to a back plane bus interface, such as a Utopia Optical Connection Level 3 (OC3) physical interface. The back plane upstream bus interface, by means of a vendor specific method (dedicated pipe, shared structure with share/grant mechanisms, etc.) sends the data to the network interface connected to the one or more content servers.
Conversely, downstream data (i.e., from the content server to the subscriber devices via the OLT 110 and an ONT) is routed to the OLT 110 by means of a vendor specific interface method (dedicated pipe, shared structure with share/grant mechanisms, etc.) from the network termination through the back plane bus interface to the APON interface of the OLT 110 (such as by a Utopia OC3 or OC Layer 12 (OC 12) physical interface). The downstream data is placed into the appropriate data slot assigned to the intended ONT of the ONTs 130-134. OAM is added to the data, the data is framed, and then sent to the optical transmitter of the OLT 110. This ATM downstream data is encrypted by the APON interface utilizing a key received from each ONT 130-134 specifically for each ONT's own data stream. In addition to the data interfacing function, the back plane bus may contain a separate management interface for equipment inventory & management, facilities management of ONT services, permanent virtual circuit (PVC) assignment, virtual circuit (VC)/virtual path (VP) cross connection management, alarm surveillance, etc.
The ONTs 130-134 are the components that terminate the optical link of the PON 120 at the customer premises. For example, the ONT 130 terminates outside of the subscriber premises 150, where the ONT 130 can be used to: provide voice content (e.g., VoIP) to/from one or more telephones 152 via a RJ11 twisted pair line; provide network data (such as Internet content) to one or more computers 154 over an Ethernet network; and provide video (either analog or digital) to one or more televisions 156. The ONTs 130-134 typically include an optical module that performs optical filtering, E/O conversion, O/E conversion, and downstream clock recovery. Downstream data received from the OLT 110 is de-framed, OAM extracted, and processed according to its content and/or destination (voice, network data, video) by the APON interface 140. For example, downstream voice content is provided to a telephone 152 (one example of a subscriber device) via a voice interface 142, downstream video content is provided to a video display 156 (another example of a subscriber device) via a video interface 146, and data content, such as data from a server on the Internet, is provided to a computer 154 (yet another example of a subscriber device) via the data interface 144. Upstream data from subscriber devices intended for the CO 104 is collected from the interfaces 142-146, multiplexed into a data stream, framed, and OAM inserted before being sent to an optical transmitter of the ONT 130. The transmitter data is adjusted into its proper system time slot by the APON interface 140 by offsetting its transmit data clock (by an amount determined by the ranging protocol) relative to the downstream clock.
While the use of optical network terminations (ONTs), also known as network interface devices (NIDs) or optical network units (ONUs), in passive optical networks provides a great deal of flexibility in data content, data transmission rates, and other design considerations, known ONTs have a number of limitations. For one, known ONTs typically implement the functionality of the APON interface 140 and the subscriber interfaces 142-146 as discrete devices often connected via a printed circuit board. However, the implementation of separate devices for the APON interface 140 and the subscriber interface 142-146 exhibits numerous disadvantages. For one, the use of separate devices on a PCB limits the reduction of the size of the ONT. Additionally, utilizing separate devices to provide PON processing functionality results in unnecessarily high power consumption, as the interfaces between the devices results in power loss due to parasitic capacitance, current leak, poorly controlled interfaces between the devices, and the like. Likewise, the signal loops on the PCB and the interconnects produce a relatively large amount of electromagnetic interference (EMI) which can interfere with the operation of the ONT. Similarly, the connections between devices and PCBs and the traces between the devices of the PCB often are somewhat unreliable, so by implementing a relatively large number of devices to provide PON processing functionality, the reliability of the ONT can be compromised. Another limitation is resource duplication between the devices, since each device often implements some common functionalities, such as memory, memory access controllers, registers, and the like. Additionally, by using numerous discrete components to implement the PON processing capability of the ONT, ONT manufacturers often must keep large inventories of the individual devices on hand.
In addition to the limitations of the physical structure of known ONTs, PON standards, such as the ITU G.983.X Recommendation, are deficient in a number of areas. For example, the ITU G.983.X Recommendation provides for a rudimentary data protection method referred to “churning.” However, the churning key used in accordance with the G.983.X Recommendation is only 24 bits long, a key length that is recognized by those skilled in the art as relatively weak. Additionally, although the G.983.X Recommendation makes provision for the dynamic allocation of bandwidth between the OLT and the ONTs, it is incumbent on the OLT to analyze the data transfer status between the OLT and the ONTs in order to modify the bandwidth allocations.
In view of the limitations of known optical network termination implementations, improved mechanisms for providing passive optical network connectivity to subscribers would be advantageous.